For soft x-ray or EUV projection lithography there is a need for a soft x-ray plasma source that can radiate within a specific wavelength of approximately 13-14 nm in the EUV part of the spectrum.
Current approaches to the development of an efficient 13-14 nm plasma light source for EUV lithography involve a high repetition-rate laser plasma source produced by the ablation of the source material from a solid target. Such a system falls short of satisfying the needed requirements for EUV lithography in several respects. First, the mass buildup of debris target material on the surface of a condenser mirror used in the lithography systems is many orders of magnitude (greater than 10.sup.3) larger than the mirror can sustain without loss of reflectivity over a reasonable operating period. Second, the spectral bandwidth of the EUV emission from the plasma is greater than the bandwidth of the multilayer mirror by a large factor, and thus the majority of the EUV emission is absorbed by the mirror. At the repetition rate of a testbed facility the thermal loading of this absorbed energy is sufficient to deform the mirror and degrade its reflectivity. Finally, no engineered scheme has yet been proposed that would guarantee continuous, unimpeded operation of a kilohertz repetition rate target facility at the required predicted cost per shot requirement of less than $10.sup.-5 /shot..sup. 3
The region of 13-14 nm is currently the preferable wavelength range for applications using soft-x-ray light because it is the region of maximum reflectivity of narrow-banded Mo:Si reflective coatings used in conjunction with near normal incidence soft-x-ray reflecting optics. There are currently no compact, efficient continuous or pulsed sources available in this spectral region. For example, the most likely source being presently considered for soft-x-my or EUV lithography operating in that wavelength region is a pulsed laser produced plasma source generated by a 1 kHz repetition rate, 1 J/pulse diode pumped solid state laser which was described by this inventor in the article entitled: "Laser-Produced Plasmas for Soft-X-Ray Projection Lithography," W. T. Silfvast, M. C. Richardson, et. al., J. Vac. Sci. Technol. B 10, 1 (1992). That laser would emit a power at 13 nm of approximately 10 W and would cost well over a million dollars, primarily due to the cost of the laser diodes required to pump the laser. Furthermore, such a laser source would have a wallplug efficiency of less than 0.1%. The lithium source disclosed in this invention would emit the same power at an electron density of 2.5.times.10.sup.17 /cm.sup.3, and would have a wallplug efficiency of the order of 10%. It would probably cost in the neighborhood of $10,000. The lithium source would be extremely compact, would be more similar to the G-line and I-line mercury arc lamps currently used for lithography, in terms of its size and efficiency, than to a laser plasma source, and would probably cost in the neighborhood of $10,000.
Other sources that have been used include synchrotons which can cost upwards of tens of millions of dollars. Synchrotons sources require large, costly facilities and arc expensive to maintain.
Thus, the need exists for a plasma source for generating pulsed or continuous emission that does not require large, costly facilities that are expensive, that are efficient in the 13 to 14 nm wavelength region and that do not produce a significant amount of debris.